High Resolution Shallow Structure of Ebao Basin Revealed With DAS Ambient Noise Tomography and Its Relation to Earthquake Ground Motion

Abstract

The Ebao Basin, which locates along the northwest segment of the Qilian-Haiyuan fault, has experienced several destructive earthquakes in past decades. A 10 km long distributed acoustic sensing (DAS) array was deployed with an existing telecom fiber-optic cable after the 2022 Mw6.6 Menyuan earthquake, which destroyed high-speed railways and highways in this region. The DAS array continuously recorded ambient noise and earthquake signals during the 5-day deployment period. Noise cross-correlation functions (NCFs) were calculated, and the Rayleigh wave signal clearly emerged on the NCFs. The phase velocities of the fundamental mode between 2.5 and 17 Hz were measured with the modified Frequency-Bessel transform method. The S-wave velocity structure along the cable, extending to a depth of 200 m, was constructed using the surface wave tomography method. It consists of three blocks, with velocity variations of up to 12%. P-wave delay times of regional earthquakes support the lateral variation among the three blocks. Both Vs30 and site amplification predicted with our S-wave velocity model correlate well with the ground motion induced by local earthquakes. Precursor and coda waves on the NCFs reveal four possible faults, and fault zone scattered waves are also observed on the record of local earthquakes.